Aqueous polymer dispersions are used in a large number of applications: for example, as base materials for paints, coatings, and adhesives, as laminating agents for paper, or as additives to building materials. Most dispersions have a solids content of from 45 to 60% by weight. At higher solids content, the viscosity rises sharply as the solids content increases. The high viscosities not only have a negative effect on processability, but rather, the high viscosity may result in coagulation of the dispersion, or in the formation of gel specks, even during polymerization.
A number of processes are known from the prior art for the preparation of highly concentrated polymer dispersions of relatively low viscosity, the majority of these processes using seed latices in order to obtain high solids contents.
EP-A 784060 relates to a process for preparing polymer dispersions having a high solids content of more than 67%, in which carboxyl-functional monomers are polymerized with further ethylenically unsaturated monomers in the presence of emulsifier and where further emulsifier is added at a monomer conversion of from 40 to 60%. WO-A 96/11234 describes a procedure in which seed latex is included in the initial charge, up to a weight fraction of 80%, and the rest of the monomer is metered in during the polymerization, without the addition of further emulsifier. In EP-A 81083 (U.S. Pat. No. 4,456,726) two polymer latices of different particle size are included in the initial charge and the monomers are polymerized subsequently. In EP-A 554832, the procedure used to prepare highly concentrated polymer dispersions involves preparing the monomers in the presence of a hydrophobic polymer and in the presence of a copolymerizable emulsifier.
The patent applications EP-A 567811 (U.S. Pat. No. 5,496,882), EP-A 567819 (U.S. Pat. No. 5,498,655), EP-A 568831 (U.S. Pat. No. 5,442,006) and EP-A 568834 (U.S. Pat. No. 5,340,859) relate to a very complicated processes for preparing highly concentrated dispersions. In EP-A 567811 an extremely finely divided latex is included at least in part in the initial charge and the monomers are polymerized under very complex process conditions. In EP-A 567819, a seed latex mixture comprising latex particles of up to 400 nm in size and latex particles of up to 100 nm in size is included in the initial charge and the monomers are polymerized under complex process conditions. EP-A 567831 relates to a process for preparing highly concentrated dispersions in which a coarsely particulate latex is included in the initial charge and a finely divided latex is metered along with the monomers. EP-A 568834, finally, relates to a process in which two seed latices, of which one includes both coarsely particulate and finely divided polymer particles, are included in the initial charge and the monomers are metered in.
Common features of the processes known to date in the prior art are that relatively complex processes are used, often in combination with a laborious seed latex technique, in order to obtain highly concentrated polymer dispersions. The object was therefore to provide a process with which dispersions virtually free of gel specks and having a solids content of more than 60% are obtainable without the need to use seed latex.
The invention provides a process for preparing aqueous polymer dispersions having a high solids content of more than 60% by polymerizing one or more ethylenically unsaturated monomers by means of free-radically initiated aqueous emulsion polymerization in the presence of from 0.1 to 5.0% by weight of emulsifier, based on the overall weight of the monomers, and in the presence of initiator, wherein the emulsifier in an amount of from 0.001 to 0.5% by weight, based on the overall weight of the water in the initial charge, is included in the initial charge together with a fraction of from 1 to 10% by weight of the monomers, based on the overall weight of the monomers, before the beginning of polymerization, and the remainder of emulsifier and the remainder of monomers are metered in after the beginning of polymerization.
Suitable monomers are one or more from the group of the vinyl esters of branched or unbranched carboxylic acids having from 1 to 12 carbon atoms, the esters of acrylic acid and methacrylic acid with branched or unbranched alcohols having 1 to 12 carbon atoms, vinylaromatics, vinyl halides, olefins, and dienes.
Preferred vinyl esters are vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl acetate, vinyl pivalate and vinyl esters of α-branched monocarboxylic acids having 9 to 11 carbon atoms, an example being VeoVa9® or VeoVa10® (trade names of Shell). Vinyl acetate is particularly preferred.
Preferred methacrylic esters or acrylic esters are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, and 2-ethylhexyl acrylate. Particular preference is given to methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate, and 2-ethylhexyl acrylate.
Preferred vinyl aromatics are styrene, methyl styrene, and vinyl toluene. The preferred vinyl halide is vinyl chloride. The preferred olefins are ethylene and propylene and the preferred dienes are 1,3-butadiene and isoprene.
If desired it is also possible to copolymerize from 0.05 to 10% by weight of auxiliary monomers, based on the overall weight of the monomer mixture. Examples of auxiliary monomers are ethylenically unsaturated mono- and dicarboxylic acids, preferably acrylic acid, methacrylic acid, fumaric acid and maleic acid; ethylenically unsaturated carboxamides and carbonitriles, preferably acrylamide and acrylonitrile; monoesters and diesters of fumaric acid and maleic acid such as the diethyl and diisopropyl esters, and also maleic anhydride, ethylenically unsaturated sulfonic acids and salts thereof, preferably vinylsulfonic acid or 2-acrylamido-2-methylpropane sulfonic acid.
Further examples are precrosslinking comonomers such as ethylenically polyunsaturated comonomers, examples being divinyl adipate, diallyl maleate, allyl methacrylate and triallyl cyanurate, or postcrosslinking comonomers, examples being acrylamidoglycolic acid (AGA), methylacrylamidoglycolic acid methyl ester (MAGME), N-methylolacrylamide (NMA), N-methylolmethacrylamide, N-methylolallylcarbamate, alkyl ethers such as the isobutoxy ether or esters of N-methylolacrylamide, of N-methylolmethacrylamide and of N-methylolallylcarbamate. Also suitable are epoxy-functional comonomers such as glycidyl methacrylate and glycidyl acrylate. Further examples are silicon-functional comonomers, such as acryloxypropyl-tri(alkoxy)- and methacryloxypropyltri(alkoxy)-silanes, vinyltrialkoxysilanes and vinylmethyldialkoxysilanes, possible examples of alkoxy groups present being ethoxy and ethoxypropylene glycol ether radicals. Mention may also be made of monomers having hydroxyl or CO groups, examples being hydroxyalkyl acrylates and methacrylates such as hydroxyethyl, hydroxypropyl or hydroxybutyl acrylate or methacrylate and also compounds such as diacetone acrylamide and acetylacetoxyethyl acrylate or methacrylate.
The most preferred auxiliary monomers are ethylenically unsaturated mono- and dicarboxylic acids, preferably acrylic acid, methacrylic acid, fumaric acid and maleic acid; ethylenically unsaturated carboxamides, preferably acrylamide and methacrylamide; and ethylenically unsaturated monomers having hydroxyl groups, preferably hydroxyethyl, hydroxypropyl or hydroxybutyl acrylate or methacrylate.
In the copolymers, the amounts in percent by weight add up in each case to 100% by weight. In general, the monomers and/or the weight fractions of the comonomers are/is selected so as to give a glass transition temperature Tg of from −70° C. to +100° C., preferably from −65° C. to +50° C. The glass transition temperature Tg of the polymers can be determined in a known manner by means of differential scanning calorimetry (DSC). The Tg can also be calculated approximately in advance by means of the Fox equation. According to Fox T. G., Bull. Am. Physics Soc. 1, 3, page 123 (1956) the following is true: 1/Tg=x1/Tg1+x2/Tg2+ . . . +xn/Tgn, where xn represents the mass fraction (% by weight/100) of the monomer n and Tgn is the glass transition temperature, in Kelvin Degrees, of the homopolymer of the monomer n. Tg values for homopolymers are listed in Polymer Handbook 2nd Edition, J. Wiley & Sons, New York (1975).
Particular preference is given to monomers and monomer mixtures which lead to homopolymers or copolymers listed below, the amounts in percent by weight, together with the auxiliary monomer fraction if appropriate, adding up to 100% by weight:
From the group of the vinyl ester polymers, vinyl acetate-ethylene copolymers having an ethylene content of from 1 to 60% by weight; vinyl acetate-acrylate copolymers with from 1 to 60% by weight of acrylate, especially n-butyl acrylate or 2-ethylhexyl acrylate, with or without from 1 to 40% by weight of ethylene.
From the group of (meth)acrylate polymers, polymers of n-butyl acrylate or 2-ethylhexylacrylate; copolymers of methyl methacrylate with n-butyl acrylate and/or 2-ethylhexyl acrylate; copolymers of methyl methacrylate with n-butylacrylate and/or ethyl acrylate; copolymers of methyl methacrylate with 1,3-butadiene.
From the group of styrene polymers, styrene-butadiene copolymers and styrene-acrylate copolymers such as styrene-n-butyl acrylate or styrene-2-ethylhexyl acrylate with a styrene content of in each case from 10 to 70% by weight.
Maximum preference is given to polymerizing the monomers or monomer mixtures just mentioned in the presence of from 0.1 to 5% by weight of one or more auxiliary monomers from the group consisting of acrylic acid, methacrylic acid, acrylamide, methacrylamide, and hydroxyethyl, hydroxypropyl and hydroxybutyl acrylate.
In the case of preparation by the emulsion polymerization process the polymerization temperature is generally from 40° C. to 100° C., preferably from 60° C. to 90° C. In the case of the copolymerization of gaseous comonomers such as ethylene, 1,3-butadiene or vinyl chloride it is also possible to operate under pressure, generally between 5 bar and 100 bar. The make-up water is included in part in the initial charge and the remainder is metered in, the metered addition possibly taking place as part of the metering of the initiator and the metering of the emulsifier. The initial amount of water is such that the emulsifier fraction in the initial charge is from 0.001 to 0.5% by weight, based on the water fraction included in the initial charge.
The polymerization is initiated by means of the initiators or redox initiator combinations which are common for emulsion polymerization. Examples of suitable organic initiators are hydroperoxides such as tert-butyl hydroperoxide, tert-butyl peroxopivalate, cumene hydroperoxide, isopropylbenzene monohydroperoxide or azo compounds such as azobisisobutyronitrile. Suitable inorganic initiators are the sodium, potassium and ammonium salts of peroxodisulfuric acid. These initiators are used generally in an amount of from 0.05 to 3% by weight, based on the overall weight of the monomers.
As redox initiators use is made of combinations of the abovementioned initiators in combination with reducing agents. Suitable reducing agents are the sulfites and bisulfites of alkali metals and of ammonium, an example being sodium sulfite, the derivatives of sulfoxylic acid such as zinc or alkali metal formaldehyde-sulfoxylates, an example being sodium hydroxymethanesulfinate, and ascorbic acid. The amount of reducing agent is preferably from 0.01 to 5.0% by weight, based on the overall weight of the monomers.
In order to control the molecular weight it is possible to use regulators during the polymerization. They are used commonly in amounts of from 0.01 to 5.0% by weight, based on the monomers to be polymerized, and are metered in separately or else as a premix with reaction components. Examples of such regulators are n-dodecyl mercaptan, tert-dodecyl mercaptan, mercaptopropionic acid, methyl mercaptopropionate, isopropanol and acetaldehyde.
The polymerization mixture is stabilized by means of emulsifiers and/or protective colloids. Preference is given to stabilization by means of emulsifiers in order to obtain a low dispersion viscosity. The overall amount of emulsifier is preferably from 0.1 to 5% by weight, in particular from 0.5 to 3% by weight, based on the overall weight of the comonomers. Suitable emulsifiers are anionic or nonionic emulsifiers or mixtures thereof, examples being:                1) Alkyl sulfates, especially those having a chain length of 8 to 18 carbon atoms, alkyl and alkylaryl ether A sulfates having 8 to 18 carbon atoms in the hydrophobic radical and from 1 to 50 ethylene oxide units.        2) Sulfonates, especially alkyl sulfonates having 8 to 18 carbon atoms, alkylaryl sulfonates having 8 to 18 carbon atoms, diesters and monoesters of sulfosuccinic acid with monofunctional alcohols or alkylphenols having 4 to 15 carbon atoms in the alkyl radical; if desired, these alcohols or alkylphenols may also be ethoxylated with from 1 to 40 ethylene oxide units.        3) Phosphoric acid partial esters and their alkali metal and ammonium salts, especially alkyl and alkylaryl phosphates having 8 to 20 carbon atoms in the organic radical, alkyl ether and alkylaryl ether phosphates having 8 to 20 carbon atoms in the alkyl or alkylaryl radical and from 1 to 50 EO units.        4) Alkyl polyglycol ethers, preferably having from 8 to 40 EO units and alkyl radicals having 8 to 20 carbon atoms.        5) Alkylaryl polyglycol ethers, preferably having from 8 to 40 EO units and 8 to 20 carbon atoms in the alkyl and aryl radicals.        6) Ethylene oxide/propylene oxide (EO/PO) block copolymers, preferably having from 8 to 40 EO and/or PO units.        
It is preferred to use mixtures of anionic emulsifiers and nonionic emulsifiers. Particular preference is given to mixtures of an diester or monoester of sulfosuccinic acid with monofunctional alcohols or alkylphenols having 4 to 15 carbon atoms in the alkyl radical, as anionic emulsifier, and as nonionic emulsifier an alkyl polyglycol ether preferably having from 8 to 40 EO units and alkyl radicals having 8 to 20 carbon atoms, in a weight ratio of from 8:1 to 1:8. In a further preferred embodiment, from 0.01 to 0.4% by weight of emulsifier, based on the overall weight of the water in the initial charge, is included in the initial charge prior to the beginning of polymerization.
If desired, the emulsifiers can also be used in a mixture with protective colloids. Examples of these are one or more protective colloids from the group consisting of partially hydrolyzed polyvinyl acetates, polyvinylpyrrolidones, carboxymethyl-, methyl-, hydroxyethyl- and hydroxypropyl-cellulose, starches, proteins, poly(meth)acrylic acid, poly(meth)acrylamide, polyvinylsulfonic acids, melamine-formaldehyde sulfonates, naphthalene-formaldehyde sulfonates, styrene-maleic acid copolymers and vinyl ether-maleic acid copolymers. If protective colloids are used, they are used preferably in an amount of from 0.01 to 1.0% by weight, based on the overall amount of the monomers. The protective colloids can be included in the initial charge prior to the beginning of polymerization, or can be metered in.
The monomers are included in the initial charge in a fraction of from 1 to 10% by weight and the remainder is metered in after the polymerization has been initiated. A preferred procedure is to include from 4 to 8% by weight in the initial charge, based in each case on the overall weight of the monomers, and to meter in the remainder. The fraction of auxiliary monomers, especially auxiliary monomers from the group of ethylenically unsaturated mono- and dicarboxylic acids, ethylenically unsaturated carboxamides and ethylenically unsaturated monomers having hydroxyl groups, is preferably not more than 5% by weight, based on the overall weight of the monomers included in the initial charge.
For initiating the polymerization, some of the initiator can be included in the initial charge and some metered in, or it can all be metered in. Preferably, the polymerization is started by heating the mixture to polymerization temperature and metering in the initiator, preferably in aqueous solution. The metered additions of emulsifier and monomers can be conducted separately or in the form of a mixture. In the case of the metered addition of mixtures of emulsifier and monomer, the procedure is to premix emulsifier and monomer in a mixer upstream of the polymerization reactor. Preferably, the remainder of emulsifier and the remainder of monomer not included in the initial charge are metered in separately from one another after the beginning of polymerization. Preferably, the metered addition is commenced from 15 minutes to 35 minutes after the beginning of polymerization.
When polymerization is at an end, residual monomer can be removed using known methods by postpolymerization, by means, for example, of postpolymerization initiated with redox catalyst. Volatile residual monomers can also be removed by means of distillation, preferably under reduced pressure, and with or without inert entraining gases such as air, nitrogen or steam being passed through or passed over the mixture.
The aqueous dispersions obtainable with the process have a solids content of more than 60% by weight, preferably from 65 to 75% by weight.
The aqueous dispersions of high solids content are suitable for use as textile binders, for use in coating compositions or in adhesive compositions. Preference is given to their use in adhesive compositions, with particular preference as pressure-sensitive adhesives.
The examples below serve to illustrate the invention further.